1. Field of the Invention
The present invention relates to a drive with a voice coil motor that executes oscillatory motion.
2. Description of the Background Art
Recent projection image displays that project images on a screen by light projection employ a device (hereinafter referred to as a “pixel shifter”) that allows incident light to pass through glass so that the glass oscillates slightly, thereby shifting the incident light in the direction of oscillation. Shifting an image on a screen by half a pixel with this pixel shifter has the effect of increasing the apparent number of pixels, thereby improving resolution.
The actuator used in this case is a voice coil motor that includes a cylindrical coil, a cup-shaped outer yoke provided to cover the outer side of the coil, a permanent magnet provided at the center of the inner face of the outer yoke and on the inner side of the coil, and an inner yoke provided at the top end of the permanent magnet.
The device that executes oscillatory motion is also a cylindrical voice coil motor. Its center of oscillation is located outside the voice coil motor and at about the midpoint position between a position where the oscillatory motion occurs and the voice-coil motor.
Examples of an apparatus using a voice coil motor as an actuator are disclosed for example in Japanese Patent Application Laid-open No. 5-326603 (cf. paragraph [0002] and FIG. 1) and the like.
A cylindrical voice coil motor generates a thrust throughout the periphery of a coil, thus achieving efficiency; however, it has the problem of difficulty in machining parts such as a cup-shaped yoke. Thus, in some cases, a voice coil motor of a different shape is used in which a yoke (center yoke) of a metal plate is provided in a hole at the center of a rectangularly-wound coil, and one magnet is provided in a position to sandwich one side of the coil. In this case, an end face of another yoke (back yoke) provided at the rear of the magnet and an end face of the center yoke provided in a hole at the center of the coil are made in contact with each other, which provides a structure that allows the magnetic flux from the magnet to circulate, thus improving the efficiency of the voice coil motor.
When the aforementioned voice coil motor with a rectangularly-wound coil is adopted, one side of the coil that is the furthest away from the center of oscillation is used as a voice coil motor in order to increase torque. As the one side of the coil used as a voice coil motor is further away from the center of oscillation, higher torque can be obtained with only a small thrust.
However, since a coil is mainly made of copper and thus concerned as one of the oscillating parts of relatively high mass, its increased distance from the center of oscillation will undesirably increase the inertia of the coil, making it difficult to increase the speed of oscillation.